Concrete box culverts may be used in a variety of applications, for example, where a throughway is desired beneath a road or other embankment. Concrete box culverts are often installed supporting a roadway at stream crossings to allow the stream to flow beneath the roadway. In many cases, concrete box culverts provide a cheaper alternative that may be more easily constructed and maintained than a bridge deck that typically requires site-specific installation and design.
In some applications, a concrete box culvert may include four sides, approximating a rectangle in cross-section. In some other applications, a three-sided box culvert may be used with two sidewalls extending orthogonally from opposite ends of a top slab. The bottom ends of the two sidewalls are each typically placed on poured footings that provide a foundation to support the three-sided culvert.
Known concrete box culverts are precast using free-standing steel reinforcing bars (i.e., rebar). The maximum span of these three-sided culverts, as measured between the inside faces of the two sidewalls, is generally limited to a range of 30-35 feet. Beyond these spans, the bending moment in the middle of the top slab becomes prohibitively large.
However, in some applications, a longer span may be needed. For example, a roadway crossing may be needed for a waterway that is more than 35 feet wide. In addition, some locales may include soils with relatively low bearing capacity. In these cases, the footings needed for the placement of a three-sided box culvert (or bridge piers) may first require the installation of piles to achieve an adequate foundation. Nonetheless, in some areas, piles may be driven dozens or even hundreds of feet into the ground before the end of the pile reaches hard strata in the soil, requiring either friction piles or an alternative design.
In some of these situations, a four-sided box culvert may be contemplated that, when installed, spreads the load of the culvert across the entire bottom slab, requiring relatively less soil bearing capacity to support the culvert. Yet, a four-sided box culvert having a relatively large span, such as 40 feet, presents logistical challenges with respect to fabrication, transportation, and installation, among other considerations. This is particularly true where the design height of the box culvert between the top and bottom slabs is also relatively large, such as 10 feet or more. For these reasons, four-sided concrete box culverts of this size are generally not entertained.